C语言基础知识-程序流程结构

　　　　　　　　　　　　C语言基础知识-程序流程结构

　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　作者：尹正杰

版权声明：原创作品，谢绝转载！否则将追究法律责任。

 

 

 

一.概述

C语言支持最基本的三种程序运行结构：顺序结构，选择结构，循环结构。
　　顺序结构：程序按顺序执行，不发生跳转。
　　选择结构：依据是否满足条件，有选择的执行相应功能。
　　循环结构：依据条件是否满足，循环多次执行某段代码。

 

二.选择结构

1>.if语句

[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# cat if_demo.c 
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:y1053419035@qq.com
*/

#include <stdio.h>

int main(void)
{
    int a = 100;
    int b = 20;
    if (a > b)
    {
        printf("a > b\n");
    }

    return 0;
}
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# gcc -o if_demo if_demo.c 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./if_demo 
a > b
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 

2>.if ... else语句

 

[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# cat if_demo2.c 
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:y1053419035@qq.com
*/

#include <stdio.h>

int main(void)
{
    int a = 100;
    int b = 200;
    if (a > b)
    {
        printf("a > b\n");
    }
    else
    {
        printf("a < b\n");
    }

    return 0;
}
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# gcc -o if_demo2 if_demo2.c 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./if_demo2 
a < b
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 

3>.if ... else if ...else语句

[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# cat if_demo3.c 
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:y1053419035@qq.com
*/

#include <stdio.h>

int main(void)
{
    int a = 300;
    int b = 300;
    if (a > b)
    {
        printf("a > b\n");
    }
    else if(a == b)
    {
        printf("a == b\n");
    }
    else
    {
        printf("a < b\n");
    }

    return 0;
}
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# gcc -o if_demo3 if_demo3.c 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./if_demo3 
a == b
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]#

4>.三目运算符【其实其内部判断条件和if相似，语法结构简单明了】

[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# cat if_demo4.c 
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:y1053419035@qq.com
*/

#include <stdio.h>

int main(void)
{
    int a = 100;
    int b = 300;
    int max;

    if (a > b)
    {
        max = a;
    }
    else
    {
        max = b;
    }
    printf("s1 = %d\n",max);

    a = 10;
    b = 20;
    max = (a > b ? a:b);　　　　　　//上面一大堆代码，我们仅仅用三目运算符一行简写。三目运算符格式为"表达式？选项1[表达式]:选项2"，即如果表达式为真，选择选项1的结果，如果为假则选择新选项2。
    printf("s2 = %d\n",max);

    return 0;
}
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# gcc -o if_demo4 if_demo4.c 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./if_demo4 
s1 = 300
s2 = 20
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 

5>.switch语句【注意：if 条件语句执行效率差，switch条件语句执行效率相对较高，但是if可以判断一个区间，而switch则只能用来判断一个值】

[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# cat switch_demo.c 
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:y1053419035@qq.com
*/

#include <stdio.h>

int main(void)
{
    char c;
    c = getchar();    //注意该方法只会接收第一个字符哟~比如你输入的是100，它只会接收第一个字符“1”

    switch(c)    //参数只能是整型变量
    {
    case '1':
        printf("OK\n");
        break;    //switch遇到break就中断了
    case '2':
        printf("not OK\n");
        break;    
    default:    //如果上面的条件都不满足，那么执行default
        printf("are u OK?\n");
    }

    return 0;
}
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# gcc -o switch_demo switch_demo.c 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./switch_demo 
1
OK
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./switch_demo 
2
not OK
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./switch_demo 
3
are u OK?
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./switch_demo 
5
are u OK?
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 

 

三.循环结构

1>.while语句

 

[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# cat while_demo.c 
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:y1053419035@qq.com
*/

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void)
{
    int a = 1;
    while(a < 10)
    {
        printf("a = %d\n",a);
        a++;
        system("sleep 0.5");
    }
    printf("程序执行完毕~\n");
    
    return EXIT_SUCCESS;
}
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# gcc -o while_demo while_demo.c 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./while_demo 
a = 1
a = 2
a = 3
a = 4
a = 5
a = 6
a = 7
a = 8
a = 9
程序执行完毕~
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 

2>.do ... while语句

 

[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# cat narcissus.c 
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:y1053419035@qq.com
*/

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void)
{
    int index = 100;
    do
    {
        int one = 0, ten=0, hundred=0;  //将一个三位数分解个位，十位，百位
        
        hundred = index / 100;        //百位

        ten = index / 10 % 10;        //十位

        one = index % 10;             //个位
        
        if (hundred * hundred * hundred + ten * ten * ten + one * one * one == index)    //各个位数的立方和等于该数本身，那么它就是一个水仙花
        {
            printf("%d是水仙花数\n",index);
        }
        
        index ++;
    }while(index < 1000);

    return EXIT_SUCCESS;
}
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# gcc -o narcissus narcissus.c 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./narcissus 
153是水仙花数
370是水仙花数
371是水仙花数
407是水仙花数
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 

3>.for循环

[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# cat for_demo.c 
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:y1053419035@qq.com
*/

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void)
{
    int index = 100;
    int sum = 0;
    
    for(index = 0;index<=100;index++)    //计算0-100之间所有数字的是总和
    {
        sum += index;
    }
    
    printf("sum = %d\n",sum);
    
    return EXIT_SUCCESS;
}
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# gcc -o for_demo for_demo.c 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./for_demo 
sum = 5050
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 

[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# cat for_demo2.c 
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:y1053419035@qq.com
*/

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void)
{
    for(int i = 100;i<1000;i++)
    {
        int one = 0, ten=0, hundred=0;  //将一个三位数分解个位，十位，百位
        
        hundred = i / 100;        //百位

        ten = i / 10 % 10;        //十位

        one = i % 10;             //个位
        
        if (hundred * hundred * hundred + ten * ten * ten + one * one * one == i)    //各个位数的立方和等于该数本身，那么它就是一个水仙花
        {
            printf("%d是水仙花数\n",i);
        }
    }
    return EXIT_SUCCESS;
}
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# gcc -o for_demo2  for_demo2.c -std=c99         #注意，在Linux系统我们编译for循环代码时需要指定"-std=c99"，否则会报错"error: ‘for’ loop initial declarations are only allowed in C99 mode"
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./for_demo2 
153是水仙花数
370是水仙花数
371是水仙花数
407是水仙花数
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 

4>.嵌套循环（循环之间可以相互嵌套）

[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# cat for_99.c 
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:y1053419035@qq.com
*/

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void)
{
    int i,j;　　　 #我们提前声明了变量i和j，如果我们在这里不声明直接在for循环里面声明也是可以的，只不过在Linux操作系统编译时，我们需要指定std的库为c99,默认使用的是c90库。否则会报错"error: ‘for’ loop initial declarations are only allowed in C99 mode"
    for(i = 1; i <= 9; i++)
    {
        for(j=1; j<=i; j++)
        {
            printf("%d x %d = %d\t",i,j,i * j);
        }
        printf("\n");
    }
    return EXIT_SUCCESS;
}
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# gcc -o for_99 for_99.c 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./for_99 
x 1 = 1    
x 1 = 2    2 x 2 = 4    
x 1 = 3    3 x 2 = 6    3 x 3 = 9    
x 1 = 4    4 x 2 = 8    4 x 3 = 12    4 x 4 = 16    
x 1 = 5    5 x 2 = 10    5 x 3 = 15    5 x 4 = 20    5 x 5 = 25    
x 1 = 6    6 x 2 = 12    6 x 3 = 18    6 x 4 = 24    6 x 5 = 30    6 x 6 = 36    
x 1 = 7    7 x 2 = 14    7 x 3 = 21    7 x 4 = 28    7 x 5 = 35    7 x 6 = 42    7 x 7 = 49    
x 1 = 8    8 x 2 = 16    8 x 3 = 24    8 x 4 = 32    8 x 5 = 40    8 x 6 = 48    8 x 7 = 56    8 x 8 = 64    
x 1 = 9    9 x 2 = 18    9 x 3 = 27    9 x 4 = 36    9 x 5 = 45    9 x 6 = 54    9 x 7 = 63    9 x 8 = 72    9 x 9 = 81    
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]#

 5>.循环语句练习一(猜年龄)

[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# cat guess_age.c 
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:y1053419035@qq.com
*/

#include <stdio.h>
#include <time.h>

int main(void)
{
    srand((unsigned int)time(NULL));        //加入随机数种子
    int num = rand()%100 + 1;
    int value;
    for(;;)
    {
        printf("猜猜看我年龄多大(请输入一个整数)>>>: ");
        scanf("%d",&value);
        if(value > num)
        {
            printf("我有那么老吗?\n");
        }
        else if(value < num)
        {
            printf("我看起来这么小吗?\n");
        }
        else
        {
            printf("太棒了，你猜对啦~\n");
            break;
        }
    }
    return 0;
}
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# gcc -o guess_age guess_age.c 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./guess_age 
猜猜看我年龄多大(请输入一个整数)>>>: 50
我看起来这么小吗?
猜猜看我年龄多大(请输入一个整数)>>>: 80
我有那么老吗?
猜猜看我年龄多大(请输入一个整数)>>>: 70
我有那么老吗?
猜猜看我年龄多大(请输入一个整数)>>>: 60
我看起来这么小吗?
猜猜看我年龄多大(请输入一个整数)>>>: 65
我有那么老吗?
猜猜看我年龄多大(请输入一个整数)>>>: 63
我看起来这么小吗?
猜猜看我年龄多大(请输入一个整数)>>>: 64
太棒了，你猜对啦~
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 

 6>.循环语句练习二(打印等腰三角形)

[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# cat isosceles_triangle.c 
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:y1053419035@qq.com
*/

#include <stdio.h>

int main(void)
{
    int row;
    printf("请输入要打印等腰三角形的行数:>>> ");
    scanf("%d",&row);
    for (int i = 1;i <= row;i++)
    {
        for (int j =1;j <= row - i;j++)
        {
            printf(" ");
        }
        
        for (int k = 1;k <= i * 2 -1;k++)
        {
            printf("*");
        }
        printf("\n");
    }
    return 0;
}
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# gcc -o isosceles_triangle isosceles_triangle.c -std=c99
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./isosceles_triangle 
请输入要打印等腰三角形的行数:>>> 3
  *
 ***
*****
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./isosceles_triangle 
请输入要打印等腰三角形的行数:>>> 10
         *
        ***
       *****
      *******
     *********
    ***********
   *************
  ***************
 *****************
*******************
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 

 

 

四.跳转语句break和contiune语句

1>.break语句

　　在switch条件语句和循环语句中都可以使用break语句：
　　　　当它出现在switch条件语句时，作用是终止某个case并跳出switch结构。
　　　　当它出现在循环语句中，作用是跳出当前内循环语句，执行后面的代码。
　　　　当它出现嵌套循环语句中，跳出最近的内层循环语句，执行后面的代码。

[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# cat do_while.c 
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:y1053419035@qq.com
*/

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void)
{
    int a = 0;
    do
    {
        a++;
        if (a == 100)
        {
            break;
        }
    }while(a);    //需要注意的是，尽管没有上面的if条件判断语句，该循环并非死循环，只是执行的次数较多而已，因为a是一个有符号int类型的数字，而int类型是有上限的

    printf("%d\n",a);
    
    return EXIT_SUCCESS;
}
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# gcc -o do_while do_while.c 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./do_while 
100
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 

2>.continue语句

　　在循环语句中，如果希望立即终止本次循环，并执行下一次循环，此时就需要使用continue语句。

[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# cat continue_demo.c 
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:y1053419035@qq.com
*/

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void)
{
    int index = 0;
    while (index < 100)
    {
        index++;
        if(index % 7 == 0 || index % 10 == 7 || index / 10 == 7)    //过滤掉带7和7的倍数的数字
        {
            continue;
        }
        printf("数字:%d\n",index);
    }
    
    return EXIT_SUCCESS;
}
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# gcc -o continue_demo continue_demo.c 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./continue_demo 
数字:1
数字:2
数字:3
数字:4
数字:5
数字:6
数字:8
数字:9
数字:10
数字:11
数字:12
数字:13
数字:15
数字:16
数字:18
数字:19
数字:20
数字:22
数字:23
数字:24
数字:25
数字:26
数字:29
数字:30
数字:31
数字:32
数字:33
数字:34
数字:36
数字:38
数字:39
数字:40
数字:41
数字:43
数字:44
数字:45
数字:46
数字:48
数字:50
数字:51
数字:52
数字:53
数字:54
数字:55
数字:58
数字:59
数字:60
数字:61
数字:62
数字:64
数字:65
数字:66
数字:68
数字:69
数字:80
数字:81
数字:82
数字:83
数字:85
数字:86
数字:88
数字:89
数字:90
数字:92
数字:93
数字:94
数字:95
数字:96
数字:99
数字:100
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 

 3>.goto语句（无条件跳转，尽量少用）

[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# cat goto_demo.c 
/*
@author :yinzhengjie
blog:http://www.cnblogs.com/yinzhengjie
EMAIL:y1053419035@qq.com
*/

#include <stdio.h>

int main(void)
{
    goto END;        //无条件跳转到END的标识
    printf("第一行字母:a\n");

END:            //我们定义的END标识,需要注意的是，这里是冒号(":")，而非分号(“;”)哟~
    printf("第二行字母:A\n");

    return 0;
}
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# gcc -o goto_demo goto_demo.c 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# ./goto_demo 
第二行字母:A
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]# 
[root@node101.yinzhengjie.org.cn /yinzhengjie/code/day002]#